This invention relates to an improved pilot operated, spool valve and, more particularly, to a pilot operated, spool valve having a spool which is moved in one direction by pilot air and in the opposite direction by a combination of mechanical biasing force and pilot air.
Heretofore there have been directional air valves utilizing a pilot operated spool. For example, in U.S. Pat. No. 4,267,862, there is disclosed a four-way directional air valve which incorporates an axially movable spool. A center exhaust port is arranged intermediate adjacent cylinder or outlet ports which, in turn, are arranged adjacent supply or inlet ports. Shifting of the spool is effected by means of pilot air in one direction and a mechanical biasing force or spring in the opposite direction.
A disadvantage with respect to such a construction is that under certain conditions the biasing spring may not provide a sufficient force to return or shift the spool once the pilot air is turned off. To overcome this problem, the spring constant of the spring may be increased. In such an instance, however, the pilot air required to move the spool against the force of the spring will increase thus making the valve useless for low pressure applications. Thus, there has existed the need and desire to provide a directional control air valve useful over a wide range of pressures.